1. Field of the Invention
This invention relates to a thin film transistor substrate, and more particularly to a thin film transistor substrate that has a structure in which a thin film transistor array is formed on color filters. Also, the present invention is directed to a method of fabricating such a thin film transistor substrate.
2. Description of the Related Art
Generally, in a liquid crystal display (LCD) having a thin film transistor array provided on color filters, a thin film transistor substrate includes color filters formed on a transparent substrate; and switching devices of thin film transistors (TFT""s) consisting of gate electrodes, a gate insulating film, an active layer, an ohmic contact layer and source and drain electrodes, and pixel electrodes, each of which is provided on the color filters.
FIG. 1 is a section view showing a structure of a conventional thin film transistor substrate having a thin film transistor array provided on color filters. Referring to FIG. 1, the thin film transistor substrate includes a black matrix 4 formed in a lattice shape on a transparent substrate 2 to divide the substrate 2 into a plurality cell areas in which color filter is to be formed and to prevent a color interference between the cell areas. On the transparent substrate 2 divided into cell units by means of the black matrix 4, red, green and blue color filters 6 for transmitting the respective red, green and blue color lights are provided. The color filters 6 are made from an acrylic or polyimide resin dispersed with pigments, and formed on the black matrix 4 to be separated from each other for the purpose of preventing a color mixture. On the surfaces of the color filters 6 is coated a smoothing layer 8 for preventing a contamination in the color filters 6 and for compensating for a step coverage or morphological difference between the red, green and blue color filters 6 formed separately to smooth the substrate. A gate electrode 10 is formed on the smoothing layer 8. The gate electrode 10 is formed by depositing a gate metal material, that is, any one of metal materials such as Al, Mo, Cr, Ta and an Al alloy or building a double layer of the metal materials on the smoothing layer 8 and thereafter patterning it. On the smoothing layer formed with the gate electrode 10, a gate insulating film 11 made from SiNx or SiOx, etc. is provided. A semiconductor layer 12 and an ohmic contact layer 14 are formed by disposing amorphous silicon (a-Si) and amorphous silicon (n+ a-Si) doped with an impurity on the gate insulating film 11 and then patterning it. Source and drain electrodes 16 and 18 are formed by depositing a metal such as Al, Mo, Cr, Ta or an Al alloy, etc. and then pattering it. A pixel electrode 22 is made by forming a protective film from a material of SiNx or SiOx, etc. and a transparent electrode material (e.g., indium tin oxide (ITO)) and thereafter patterning them.
As described above, the conventional thin film transistor substrate inevitably requires the smoothing layer 8 so as to compensate for step coverage between the red, green and blue color filters 6. However, the smoothing layer 8 does not make a little effect to a step coverage removal in the thin film transistor formed at the upper portion of the smoothing layer 8. In the thin film transistor, a badness may occur at the layers (i.e., the semiconductor layer and the source and drain electrodes) disposed in the post process due to a step coverage in the gate electrode. Upon manufacturing of a large-dimension panel, it is necessary to form the gate electrode into a double metal layer for the sake of a good signal transmission. However, since a step coverage in the gate electrode is enlarged in the case of forming the gate electrode into a double metal layer, it becomes difficult to form a double layer of gate electrode and a material selection of the gate electrode is limited.
Accordingly, it is an object of the present invention to provide a thin film transistor substrate wherein a gate electrode is provided within a smoothing layer so as to compensate for step coverage in a thin film transistor.
A further object of the present invention is to provide a method of fabricating a thin film transistor substrate wherein a gate electrode is provided within a smoothing layer so as to compensate for step coverage in a thin film transistor.
In order to achieve these and other objects of the invention, a thin film transistor substrate according to one aspect of the present invention includes a thin film transistor formed on color filters; and a smoothing layer compensating for a step coverage between the color filters and being provided with a recess in which a gate electrode of the thin film transistor is to be formed.
A method of fabricating a thin film transistor substrate according to another aspect of the present invention includes the steps of forming color filters on a transparent substrate; forming a smoothing layer on the color filters and then pattering it to thereby define a recess in which a gate electrode of the thin film transistor is to be formed; and forming the thin film transistor on the smoothing layer.